1. Field of the Invention:
The invention relates to polymer mixtures. It concerns especially those comprising an aromatic polycarbonate, particularly the polycarbonate of Bisphenol A, and a methyl methacrylate copolymer, which are distinguished by high transparency.
2. Discussion of the Background:
It is known how to mix aromatic polycarbonates, especially polycarbonates of Bisphenol A, with other polymers, to obtain combinations that may be multiphased or incompatible polymer mixtures, so-called polymer blends, or homogeneous, compatible polymer mixtures. Compatible polymer mixtures are also called polymer alloys.
Polymethyl methacrylate molding material, a glass-clear, transparent plastic, does not give homogeneous, transparent alloys with the polycarbonate of Bisphenol A, also a transparent plastic, but rather mixtures with pearly luster, as disclosed by Japanese Patent 72 16 063.
According to German 22 64 268, an improved polycarbonate molding material is obtained when an acrylic polymer of low molecular weight which is a copolymer consisting of 90 to 75 wt.% methyl methacrylate and 10 to 25 wt.% of an alkyl acrylate with the formula ##STR1## with X equal to H or CH.sub.3 and R being an organic group with 4 to 12 carbon atoms, is added to the polycarbonate. The amount of copolymer additive can be 0.01 to approximately 50 wt.% of the weight of the polycarbonate composition. The melt viscosity of the polycarbonate or of the mixture is progressively reduced with increasing addition of copolymer, without impairing the transparency.
Therefore, these copolymers are polymeric plasticizers whose molecular weight must be below 15,000 to produce the compatibility described, on the basis of the present inventor's experiments with appropriate copolymers. To produce polymer alloys that have properties of industrial interest in the range of high polymethacrylate fractions, however, such copolymers are completely unsuitable because of the familiar degradation of the mechanical properties in the molecular weight range below 100,000, particularly below 50,000 (see Plastics Manual, Volume IX, Vieweg/Esser: Polymethacrylates, Pages 112 ff).
Compatible polymer mixtures that consist of a polycarbonate such as Bisphenol A polycarbonate and a copolymer of monomeric esters of acrylic and/or methacrylic acid with C.sub.1 --C.sub.10 alcohols and a UV-absorbing monomer of the formula ##STR2## in which R.sub.1 stands for hydrogen or a methyl group and Y stands for oxygen or an NR2 group with R2 standing for hydrogen or alkyl group, and Z stands for a UV-absorbing group, namely a 2-hydroxybenzotriazole group, a 2-hydroxybenzophenone or acetophenone group, or an .alpha.-cyano-.beta.,.beta.-diphenyl group, are described in German Application P 35 18 538.4.
Such polymer mixtures of polycarbonate and copolymers of methyl methacrylate and monomers with pronounced absorbing power for ultraviolet radiation, which can also be processed as thermoplastics and that are used as optical filters to improve the resistance to light of plastics, for example, especially as coatings, are polymer mixtures with only very special uses and are prohibitively expensive for widely usable compositions that can be processed as thermoplastics because of the high cost of the UV absorber incorporated in them. German Application P 36 32 946.0, describes methyl methacrylate copolymers with methacrylamides as comonomers that are substituted on the amide nitrogen with a cyclic organic group that also has no pronounced UV-absorbing power, which can be processed as thermoplastics. These copolymers form transparent polymer mixtures with polycarbonates, especially Bisphenol A polycarbonate, which can be processed as thermoplastics.
Thermoplastic molding materials such as polymer mixtures of a polycarbonate, a copolymer consisting of styrene, methyl methacrylate, and N-phenylmaleimide, and a graft polymer of methyl methacrylate on rubber, are not compatible according to European 173 146. Likewise, the polymer mixtures disclosed by European 144 231, consisting of a polycarbonate and a copolymer of methyl methacrylate/N-phenylmaleimide, and/or an EPDM-graft-methyl methacrylate/N-phenylmaleimide copolymer, are not fully compatible and are thus polymer blends.
A need continues to exist for compatible polymer mixtures that can be processed as molding materials and which are comprised of polycarbonate and a polymer that consists of high proportions of methyl methacrylate.